THE PHYSICAL STRUCTURE OF ORION-KL ON 2500-AU SCALES USING THE K-DOUBLET TRANSITIONS OF FORMALDEHYDE

The physical characteristics of a molecular cloud which relate most cl osely to star formation within it occur on the finest spatial scales. Generally, several transitions of a specific molecule must be mapped t o securely determine molecular cloud physics. Toward this goal, interf erometric observations of the 1(10) --> 1(11) and 5(14) --> 5(15) tran sitions of H2CO have been made toward the Orion-KL molecular cloud. Wi th synthesized beamwidths of 5.1'' (2400 AU) and 7.6'' (3500 AU), resp ectively, we identify emission from the '' hot core,'' ''compact ridge ,'' and ''northern cloud'' regions. We also detect 1(10) <-- 1(11) H2C O absorption toward the ''Orion-S'' region. These 1(10) --> 1(11) and 5(14 --> 5(15) emission measurements have been combined with 6'' resol ution measurements of the 2(11) --> 2(12) transition of H2CO (Mangum e t al. 1990) in a spherical large velocity gradient model of the H2CO e xcitation to derive the H-2 density and H2CO column density in the hot core, compact ridge, and northern cloud. Typical peak densities lie i n the range (3-8) x 10(5) cm-3, with H2CO column densities in the rang e 10(16)-10(17) cm-2. Highest spatial densities but lowest column dens ities occur in the northern cloud, while lowest spatial densities but highest column densities characterize the compact ridge. A critical di scussion of H2CO as a spatial density probe demonstrates that the K-do ublet transitions provide an excellent spatial density probe, attainin g substantial optical depths only at the highest column densities.